High strength and oxidation resistant materials are often used in components for gas turbines. For example, nickel-based and cobalt-based superalloys are often used for buckets, blades, nozzles, or other components within gas turbines. Such superalloys can have poor weldability. Once formed, the properties required for these superalloys to survive in gas turbine applications, such as for example creep-rupture resistance, are developed by heat treatments that develop precipitates throughout the superalloy. These precipitates include gamma prime (γ′) and gamma double prime (γ″) uniformly distributed throughout a gamma matrix. These phases are developed by heat treatments while avoiding the formation of deleterious phases such as topologically close-packed (TCP) phases, Laves phases and other undesirable phases depending on the specific composition of the superalloy. The presence of these phases contributes to low ductility, brittleness, loss of other mechanical properties and possible detrimental effect on environmental properties such as corrosion resistance. After the microstructure has been developed in gas turbine articles or components comprising superalloys, care must be taken not to damage such articles or components, as the articles or components cannot be readily repaired without detrimentally affecting the developed microstructure using available procedures.
For example, features are added to components of gas turbines having nickel-based and cobalt-based superalloys by machining. Machining results in a loss of material and can be expensive. Machining can take a relatively long time, particularly for parts having complex geometries, and any mis-machining of the components and can result in scrappage of the articles or components.
In another example, parts taken out of service may experience service related wear or damage that cannot be restored to their original configuration using available techniques. The lack of a suitable repair procedure results in the scrappage of articles or components removed from service because of damage to key features or simple failure to meet dimensional criteria for continued use in gas turbine engines.
Features may be added to components of gas turbines comprising precipitation-hardenable nickel-based and cobalt-based superalloys by joining processes, such as brazing or welding. Each of these processes suffers from various infirmities, particularly once the precipitates have been developed in the superalloy. In order to be effective, both brazing and welding require the application of heat to the article or component being repaired, and this application of heat has a detrimental effect on the developed precipitates, altering the precipitates and frequently resulting in the formation of additional detrimental precipitates such as TCP phases, Laves phases and/or other detrimental phases depending on the specific composition of the superalloy.
Brazing is generally performed at elevated temperatures below the melting point of the base material but above the melting point of the braze material. Brazing is detrimental due to the heat required to successfully join, for example, a repair area or repair material to a fully developed article of component. As used herein, a fully developed article or component is one in which a precipitation-hardened microstructure, without the inclusion of detrimental phases, is developed throughout the article or component. Brazing uses a braze material that can introduce different considerations. For example, distortion sometimes occurs during the brazing process for certain braze materials. The braze metal, or at least one braze metal component of a multiple braze metal composition, has a lower melting temperature than the base materials that are to be joined. Multiple component braze materials are typically a mixture of metal particles, each of two or more different alloys, one of which includes a melting point depressant such as boron or silicon to achieve a lower melting point. In addition, brazing can require a higher level of sophistication from the operator. Brazing also requires placing an entire component into a vacuum furnace and heating of the entire component during the braze cycle, thereby limiting applicability to size constrained and/or temperature-sensitive applications. Not only is the entire component placed into the furnace and heated to an elevated temperature at which the lower melting point component melts, usually close to the melting point of the materials to be joined, but the component also is held at an elevated temperature while diffusion occurs between the low melt components and the high melt components so that the melting point depressant additions diffuse away from the braze zone and raise the melting temperature of the braze zone. As a result segregation in the braze zone is reduced and a more homogenous chemical composition is developed between what were the low melt and high melt components. However, these elevated temperature treatments undesirably affect the previously developed precipitates, resulting in their dissolution or growth, depending on the temperatures, grain growth in multiple-grained components, possible nucleation of new grains in previous single crystal components and formation of detrimental phases.
Welding similarly adds heat to affect the joining of new material to the previously fully developed article or component. In welding, sufficient heat must be added to melt the filler material. This heat not only melts the filler material, but also a localized portion of the component, the portion of the component being melted referred to as the base metal and the entire melted material being referred to as weld metal, but, as is well known, results in regions of the component or article adjacent to the weld metal being heated to elevated temperatures as heat conducts away from the weld metal. This region is referred to as the heat affected zone. The microstructure of the weld metal will have few or no precipitates, as the cooled weld metal has not been exposed to a heat treatment for the development of precipitates. The precipitates in the heat affected zone may experience growth due to elevated temperatures from the heat being conducted through it. In addition, this heat may be sufficient to cause the nucleation or precipitation of detrimental phases discussed above. Further heat treatment for stress relief or to attempt to develop precipitates in the weld metal will only exacerbate the problem of precipitate growth and detrimental phase formation in the heat affected zone but will likely extend the problem to the non-heat affected zone. None of this analysis even considers the effect of grain growth on the welded components due to welding and subsequent heat treatment.
Some components further include dimensional requirements for sealing and/or locating of sub-components. For example, some components have identifiable features on the sub-components. In some cases, such sub-components are re-designed due to the limitations of the selected joining process.
A process for repairing articles of components that enable the addition of material to the joined article while eliminating or minimizing one or more of the above drawbacks of prior art methods would be desirable in the art.